The requirements for high areal recording density impose increasingly greater requirements on thin film magnetic recording media in terms of coercivity, magnetic remanence (Mr), coercivity squareness (S*), low medium noise and narrow track recording performance. It is extremely difficult to produce a magnetic recording medium satisfying such demanding requirements, particularly a high areal recording density magnetic rigid disk medium for longitudinal recording.
The linear recording density can be increased by increasing the coercivity of the magnetic recording medium. However, this objective can only be accomplished by decreasing the medium noise, as by maintaining very fine magnetically noncoupled grains. Medium noise is a dominant factor restricting increased recording density of high density magnetic hard disk drives and, hence, has become a road barrier to ultra-high areal density recording. Medium noise in thin films is attributed primarily to inhomogeneous grain size and intergranular exchange coupling among magnetic grains. Therefore, in order to increase linear density, medium noise must be minimized by suitable microstructure control.
A conventional longitudinal recording disk medium is depicted in FIG. 1 and typically comprises a non-magnetic substrate 10 having sequentially deposited thereon a plating 11, such as a plating of amorphous nickel-phosphorous (NiP), and underlayer 12, such as chromium (Cr) or a Cr-alloy, a magnetic layer 13, typically comprising a cobalt (Co)-based alloy, and a protective overcoat 14, typically containing carbon. Conventional practices also comprise bonding a lubricant topcoat (not shown) to the protective overcoat. Underlayer 12, magnetic layer 13 and protective overcoat 14 are typically deposited by sputtering techniques. The Co-base alloy magnetic layer deposited by conventional techniques normally comprises polycrystallites epitaxially grown on the polycrystal Cr or Cr-alloy underlayer.
A substrate material conventionally employed in producing magnetic recording rigid disks comprises an aluminum-magnesium (Al-Mg) alloy. Such Al-Mg alloys are typically electrolessly plated with a layer of NiP at a thickness of about 15 microns to increase the hardness of the substrates, thereby providing a suitable surface for polishing to provide the requisite surface roughness or texture.
Werner et al., U.S. Pat. No. 4,900,397, proposed the use of Radio Frequency (RF) sputter etching to remove surface deposits on a conventional NiP coating of Al-alloy substrates, followed by oxidization, to improve adhesion of a Cr underlayer. Doerner et al., U.S. Pat. No. 5,302,434, found it difficult to obtain high coercivity on superpolished untextured NiP coated substrates, and proposed annealing in air to form a nickel oxide film on the surface of the NiP coating for enhanced coercivity. The smooth surface of the polished NiP layer was maintained through subsequent layers. The nickel oxide film was also said to reduce modulation by altering the crystallographic orientation of the underlayer and magnetic layer. Aluminum alloy substrates of magnetic recording disks exhibit an undesirable tendency to warp into potato chip shape when very thin disks are formed to satisfy increasing demands for reducing the physical size of disk drives, especially for portable computers. Such warping adversely affects control of spacing between the disk and transducer head. Moreover, aluminum alloy substrates do not possess sufficient hardness to satisfy current disk drive shock impact requirements.
Alternate substrate materials have been employed for magnetic recording media, such as glasses, e.g., an amorphous glass, and glass-ceramic materials which comprise a mixture of amorphous and crystalline phases. Glass-ceramic materials do not normally exhibit a crystalline surface. Glasses and glass-ceramics generally exhibit high resistance to shocks. The use of glass-based materials, such as glass-ceramic materials, is disclosed by Hoover et al., U.S. Pat. No. 5,273,834.
Conventional methods for manufacturing a magnetic recording medium with a glass or glass-ceramic substrate comprise applying a seed layer between the substrate and underlayer. Such magnetic recording media with glass or glass-ceramic substrates are commercially available from different manufacturers with different seed layer materials to reduce the high thermal emissivity of such glass and glass-ceramic substrates, and to influence the crystallographic orientation of subsequently deposited underlayers and magnetic layers. Such conventional seed layer materials also include NiP which is typically sputter deposited on the surface of the glass or glass-ceramic substrate at a thickness of about 500 .ANG..
Conventional magnetic recording media comprising a glass or glass-ceramic substrate having NiP sputter thereon also comprise, sequentially deposited thereon, a Cr or Cr-alloy underlayer at an appropriate thickness, e.g., about 550 .ANG., a magnetic layer such as Co-Cr-platinum (Pt)-tantalum (Ta) at an appropriate thickness, e.g., about 350 .ANG., and a protective carbon overcoat at an appropriate thickness, e.g., about 150 .ANG.. Conventional Cr-alloy underlayers comprise vanadium (V) or titanium (Ti). Other conventional magnetic layers are CoCrTa, CoCrPtB, CoCrPt and CoNiCr. The seed layer, underlayer, and magnetic layer are conventionally sequentially sputter deposited on the glass or glass-ceramic substrate in an inert gas atmosphere, such as an atmosphere of pure argon. A conventional protective carbon overcoat is typically deposited in a mixture of argon with nitrogen, hydrogen or ethylene. Conventional lubricant topcoats are typically about 200 .ANG. thick.
Magnetic films exhibiting a bicrystal cluster microstructure are expected to exhibit high coercivity, low noise and high remanent squareness. In co-pending application Ser. No. 08/586,571, filed on Jan. 16, 1996, a magnetic recording medium is disclosed comprising a glass or glass-ceramic substrate and a magnetic layer exhibiting a bicrystal cluster microstructure. The formation of a bicrystal cluster microstructure is induced by oxidizing the surface of a seed layer so that the underlayer subsequently deposited thereon exhibits a (200) crystallographic orientation which, in turn, induces a bicrystal cluster microstructure in a magnetic alloy layer deposited and epitaxially grown on the underlayer.
Co-pending application Ser. No. 08/586,529, filed on Jan. 16, 1996, now U.S. Pat. No. 5,733,370 discloses a method of manufacturing a magnetic recording medium comprising a glass or glass-ceramic substrate and a magnetic layer exhibiting a bicrystal cluster microstructure. The disclosed method comprises sputter depositing an NiP seed layer on a glass or glass-ceramic substrate and subsequently oxidizing the deposited NiP seed layer. The oxidized upper seed layer surface induces the subsequently deposited underlayer to exhibit a (200) crystallographic orientation which, in turn, induces the magnetic alloy layer deposited and epitaxially grown on the underlayer to exhibit a bicrystal cluster microstructure. The magnetic recording media disclosed in co-pending application Ser. Nos. 08/586,571 and 08/586,529 now U.S. Pat. No. 5,733,370 exhibit high coercivity, low Mrt and low noise, thereby rendering them particularly suitable for longitudinal recording. In copending application Ser. No. 08/699,759, a method is disclosed for manufacturing a magnetic recording medium wherein a partially oxidized seed layer is deposited on a glass or glass-ceramic substrate. The subsequently deposited Cr or Cr-alloy underlayer thereon exhibits a (200) dominant crystallographic orientation. The entire disclosures of co-pending application Ser. Nos. 08/586,571, 08/586,529 and 08/699,759 are incorporated by reference herein.
There exists, however, a need for a magnetic rigid disk media for longitudinal recording exhibiting low medium noise and high coercivity, and for an efficient, cost-effective method of manufacturing such magnetic recording media with high production throughput.